JP2910956B2 - Method and apparatus for inspecting surface for defects having repetitive patterns - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus and a method for inspecting defects in a product having a regular pattern, and more particularly to a dark field imaging technique used for inspecting a semiconductor wafer having a fine repeating pattern. About.

[0002]

2. Description of the Related Art Detection of fine particles and defects on patterned wafers is an important problem in the semiconductor industry. Contaminant particles on the surface of a semiconductor wafer create undesirable conduction paths for integrated circuits formed on the wafer. Defects in the photolithographic pattern used to create multiple integrated circuits yield defective or substandard devices. It is important to identify any defect types or features of the integrated circuit at various stages of processing so that the defects do not adversely affect yield. In the prior art, there are many ways to find defects and contaminant particles on the surface of a semiconductor wafer. In general, these methods can be divided into three categories. Spatially filtered brightfield imaging technology, analysis of brightfield images and low angle darkfield imaging technology.

[0003] Such prior art is disclosed, for example, in US Patent No. 4,771,468. A feature of this technology is a bright-field inspection technology that recognizes contaminant particles and pattern defects through image processing of dark-field images (without the spatial filtering method). The basis of the algorithm is to compare the corresponding pixel with the array element on the circuit. If the center pixel matches the corresponding left and right pixels, there is no defect.
Additional processing checks for instabilities that occur when pixels are next to lines or on rough surfaces, when there is interference noise from the camera, or when there is a systematic change in the optical properties of the part under inspection. Used for When the information created by such a system is about the same as the information created by this embodiment of the present invention, the key difference from the system described in the referenced patent is relatively expensive (electronic imaging). Slower (due to the complexity of the video analyzer) (because of the analyzer).

[0004] Another prior art system is disclosed in US Pat.
No. 806774. This technology is incorporated herein by reference for optics teaching and semiconductor wafer inspection. This system uses a Fourier transform lens and an inverse Fourier transform lens on the optical axis. This system forms a dark-field image of an area on a wafer on a remote image plane. In this system,
The spatial frequency corresponding to the repetitive pattern is selectively attenuated in the dark field image by inserting a Fourier transform plane spatial filter between the lenses. The resulting image highlights irregularities on the surface of the integrated circuit, such as those resulting from contaminant particles or defects in the pattern.

However, in this system, the wafer is illuminated via a Fourier transform lens. That is, light dispersed and reflected by the lens is added to the image, increasing the background illumination level. In addition, relatively strong zero-order reflected light from the wafer also passes through a Fourier transform lens that produces additional background illumination. To effectively obstruct the illuminated pattern, the Fourier spatial filter used in this system is optically dense. This reduces the amount of light that passes through the filter. All combinations of these effects reduce the sensitivity of the defect inspection system.

Another type of defect detection system is shown in US Pat. No. 4,772,126. It is incorporated herein by reference for teachings on optics and inspection of semiconductor wafers. In the system described in this patent, a semiconductor wafer is illuminated at a critical line-of-sight angle between 0 and 5 degrees of the wafer surface. The illumination light is directed to strike the wafer at an angle of about 45 degrees with respect to the lines of the rectangular pattern. Further, the light beam is scanned over the surface of the wafer by a scanning galvanometer.
Light reflected at an angle substantially normal to the wafer surface is collected by a video camera on the wafer. This is a conventional marginal line-of-sight configuration, where the rays preferentially illuminate the particles that spread on the surface of the wafer. The light reflected by these particles is collected by a camera. The surface of the wafer acts as a mirror for this low angle incident light so that particles that extend well away from the surface receive a higher level of illumination. Further, this patent suggests the use of a Fourier spatial filter to attenuate spatial frequencies corresponding to repetitive patterns from images collected by a video camera.

While this system is sufficient for examining particulates on the surface of a wafer, it is not efficient for detecting smaller particulates that are embedded in the surface features of the wafer or cause repeated pattern errors. . Because these features are at or slightly below the surface of the wafer, they are not illuminated by low-angle rays and do not appear in dark-field images. Furthermore, as the illuminating light beam is scanned over the surface of the wafer, the spatial frequency components oscillate at the scanning frequency in the Fourier plane. As a result, Fourier filters that filter out these spatial frequencies must be opaque over a larger area than if no rays were scanned. This type of filter needs to reduce the illumination difference between small particles and defects on the one hand and the background illumination level on the other hand. This reduces the sensitivity of the device to such defects when a Fourier spatial filter is used.

[0008]

SUMMARY OF THE INVENTION The present invention is directed to a method for illuminating a product having a repeating pattern with a monochromatic ray at an angle of penetration relative to the wafer surface between 8 degrees and a maximum angle determined by the imaging lens system. Dark-field inspection system. This angle is limited by the constraints that the inspection system generates a dark field image.
Light scattered at an angle approximately normal to the surface is collected by the lens system. This lens system spatially filters the collected light to substantially attenuate the spatial frequency components corresponding to the repeating pattern. The rest of the light
Collected by the lens system to form an image in which particles and defects in the provisional pattern are enhanced.

An inspection system according to the present invention utilizes a combination of dark field imaging and Fourier spatial filtration to detect large and small particles and pattern defects in products such as semiconductor wafers that exhibit elaborately characterized repetitive patterns. The principle of Fourier spatial filtration is described in the paper "Inspection of LSWatkins".
Integrated Circuit Photomasks with Intensity Spati
al Filters "IEEE Bulletin, vol 57, No. 9, 9/69. This article is incorporated herein by reference. Although described in the context of a semiconductor wafer inspection system, it is contemplated that it can be used to inspect other products having a repeating pattern, such as photolithographic masks used to manufacture semiconductor devices. The system is also used to inspect liquid crystal devices and shadow masks of color cathode ray tubes.

In an embodiment of the present invention, a semiconductor wafer having a pattern etched thereon is illuminated by a monochrome light source such as a laser 11 at an angle of penetration θ with respect to the wafer surface. This angle is selected to illuminate the wafer sufficiently for the pattern to be inspected, but to block low spatial frequency components of the pattern from the imaging system (i.e., those closest to mirror-like reflected rays). Further, the angle φ of the pattern plane is selected to further reduce the intensity of light scattered from the pattern features. This angle φ is the angle that the incident light forms with respect to the rectangular features of the pattern. It has been found that for a pattern having substantially rectangular features, when the angle φ is 45 degrees, the intensity of light scattered from the pattern features is minimized.

This vertically scattered light is collected by an optical system that includes a spatial filter 120 that produces a dark field image. That is, features of the surface associated with the image where the background appears dark are illuminated. The spatial filter is selected to strongly attenuate spatial frequency components caused by patterns entering the optical imaging system. The filtered light is collected in an image captured by video camera 128 for analysis by computer 130. The computer executes an algorithm to identify and analyze the defect and determine the characteristics and size of the defect.

These different optical systems are illustrated in various embodiments of the present invention. The first system uses a standard Fourier transform and an inverse Fourier transform lens with a Fourier spatial filter to produce a filtered image. In the second embodiment, a 1: 1 lens is used as an objective lens. This lens is combined with a Fourier spatial filter and also attenuates the spatial frequency components of the dark field image. Third
An embodiment of the invention uses a standard infinity-corrected microscope objective, a defocused relay lens, and a tube lens connected to a Fourier filter to magnify the dark field image collected from the wafer and spatially filter the image. To highlight defects in the pattern.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention is shown in FIG.
In this figure and in FIGS. 2 and 3, the path of the scattered light is illustrated by the trace of the light beam. The light is shown as a dotted line. These lines represent the paths of the diffracted parallel rays. These lines do not represent points of the object being imaged. This notation is used to more clearly indicate the spatial filtration of light.

In all of the examples, conventional helium-
A neon laser 110 creates a parallel beam of monochrome light that is reflected by a surface mirror 112 at an angle θ of 45 degrees with respect to the surface of the semiconductor wafer being inspected. The light beam reflected by the mirror 112 is expanded by a conventional beam expander 114 into a wider parallel light beam. The light beam provided by the expander 114 is polarized by a polarizing filter 115. Instead, filter 1
15 is removed if a laser 110 that produces polarized light is selected.

The light beam provided by the system illuminates a portion of the wafer 116. A major portion of the light ray is reflected from the field of view of the imaging system as shown by the solid line. However, some rays are scattered as indicated by the dotted lines. Light rays scattered in a direction substantially perpendicular to the wafer surface are collected by a Fourier transform lens 118 and positioned one focal length (f) from the top surface of the wafer 116. This lens performs a Fourier transform of the parallel light reflected from the surface of the wafer 116 on a surface located one focal length behind the lens. This plane is called the Fourier transform plane of the system.

The Fourier spatial filter 120 is arranged on the Fourier transform plane so as to prevent spatial frequency components of scattered light corresponding to the repetitive pattern. FIG. 5 shows the filter 120.
Is shown in The filter used depends on the spatial frequency component of the pattern to be inspected. A suitable spatial filter is formed by placing an unexposed photographic film in the Fourier plane and illuminating the sample wafer with laser 110. A negative of the Fourier plane image is obtained by developing the exposed film and can be used as a Fourier filter on the wafer corresponding to the sample wafer. Alternatively, a spatial light modulator can be used as spatial filter 120.

The spatially filtered light beam provided by Fourier filter 120 is converted back to a parallel light beam by a second lens 122 located one focal length behind filter 120. This ray forms a dark-field image of the illuminated portion of the wafer at the image plane 124. The magnifying optical mechanism 126 magnifies the image, and the image is converted into an electric signal by a normal video camera 128. The signal from the camera 128 is converted into digital data and supplied to the computer 130. This computer 13
0 analyzes the data to identify and classify pattern irregularities on the surface of the wafer.

An additional lens element (not shown) can be used to image the Fourier transform plane on a video camera. This image is transferred to a spatial light modulator, which configures it as a spatial filter 120. Alternatively, this image can be used to determine the proper match of the Fourier filter for the photograph. As shown in FIG. 2, the wafer 116 is positioned so that the incident light beam enters the surface at a 45 degree angle at the wafer surface with respect to the vertical line dominant at the pattern surface. This angular position has been selected to minimize the intensity of light scattered by the pattern into the lens system.

The system of FIG. 1 extends the defect of the pattern with respect to the pattern in a number of ways. First,
The lighting is selected to create a dark field image, and the image is spatially filtered. Dark-field imaging technology emphasizes features on the wafer with respect to the background and, through spatial filtration,
An image component having a spatial frequency related to the repeating pattern is attenuated. Second, the wafer is illuminated at an angle of incidence θ with respect to its surface and an angle of inclination φ with respect to pattern lines. The effect of this technique is shown in FIG. In this figure,
The graph of the Fourier transform of the repeating pattern is two circles representing the direct viewing numerical aperture 410 when the wafer is illuminated perpendicular to its surface and the direct viewing numerical aperture 412 when the illumination is deviated from orthogonal by angles θ and φ. Can be stacked. The numerical aperture is
Includes light collected by the system to produce the final image.

As shown in FIG. 6, when the illumination is orthogonal, a significant portion of the direct-view aperture is accurately reflected corresponding to the zero order Fourier frequency spectrum of the pattern and the strongest portion of the diffracted ray. Dominated by rays. These components of the image are prevented by Fourier filter technology used in bright field imaging devices such as the above-mentioned US Pat. No. 4,806,774. However, this Fourier filter needs to prevent much of the light at the direct viewing numerical aperture and create an image with relatively low contrast. Conversely, when the illumination source is moved by the angles θ and φ in the present invention, the direct-view aperture is moved to a space where the spatial frequency spectrum is relatively empty. Images acquired through this aperture require less filtering and remove artifacts associated with the repeating pattern. This increases the contrast of the image and the wafer defects are more emphasized by this system than by the spatially filtered brightfield imaging system. Further, as the power of the illumination due to the pattern passing through the lens system is reduced, the scattering generated at each element of the lens system is also reduced. For this reason, the level of the background ray of the dark field image is reduced.

Finally, the illumination applied to the wafer is polarized. Depending on the roughness of the surface features, it is desirable that p-polarized or s-polarized light further reduce the visibility of pattern features in dark-field images. In many cases, s-polarized light is preferred because the zero electromagnetic surface of s-polarized light suppresses surface roughness. However, it is desirable to use p-polarized light in some wafers. An example of this type of wafer is a wafer having a feature of grooves formed by etching a layer of silicon nitride and having a thickness of about 1/4 wavelength grown or deposited on a silicon substrate.

With the system shown in FIG. 1, relatively small particles and defects in the pattern (about 0.34 microns) can be distinguished. However, the system
Relatively expensive and difficult to maintain alignment. The system shown in FIGS. 3 and 4 is less expensive and
Easy to use than the system shown.

The system shown in FIG.
Same as that shown in FIG. 1 except that 8 and 122 have been replaced with a single 1: 1 lens 210. Lens 21
0 is used as an objective and Fourier transform lens to magnify the area under inspection. Wafer is lens 21
When positioned at 0 to 2 focal points (2f), the image of the wafer is formed at an image plane positioned 2f behind the lens. Further, the Fourier transform plane is one focal length (f) behind the lens. The performance of this system is equivalent to that of the system shown in FIG. 1, but with one less component in the optical system.

The system shown in FIG. 4 uses a conventional infinity-corrected microscope objective 310, an afocal relay lens (314 and 316) as commonly used in periscopes, and a tube lens 318. This tube lens 318 is used in place of the custom lens 118, lens 112 and magnifying optics 126 of the system of FIG. This adds more components to the system, but at a lower price. The objective lens of the microscope is not as expensive as the custom-made Fourier lens 118, and the tolerance between the relay lens and the tube lens is looser than the lens 122 and the magnifying optical mechanism 126. This is because the microscope objective lens magnifies the image of the semiconductor wafer. A problem with using the microscope objective 310 in a system that performs a Fourier spatial filtering operation is gaining access to the Fourier plane (ie, the back focal plane 31 of the microscope objective).
4). In most microscope objectives, this plane is located in the objective lens system. The defocused relay lens systems 314 and 316 provide the accessible focal plane of the system.

As shown in FIG. 4, lens 314 is positioned one focal length from the inaccessible rear focal plane of microscope objective 310. The second lens 316 of the relay lens system forms a Fourier plane an equivalent distance after the defocused relay lens system. The Fourier filter 120 is located at this position.

As detailed above, the microscope objective 310 is an infinitely modified lens. Optical systems using this type of lens use a tube lens 318 to achieve a standard optical tube distance of, for example, 160 mm. As described above, in the imaging system shown in FIG. 4, the lens constraints after the microscope objective 310 are:
Loose compared to the optical system used in other embodiments of the present invention. For example, an image of a wafer is a microscope objective lens 3
10, the resolution requirement of the relay lens system is relaxed (ie, the relay lens 31).
4 and 316 need not be diffraction limited. Furthermore, the numerical aperture (NA) of relay lens systems 314 and 316 is only 1 / M times that of objective lens 130. Here, M is a magnification factor of the lens 130. However, these lenses are better high quality imaging lenses such as those used in 35 mm cameras.

Although all of the above described embodiments of the present invention have employed collimated light sources, divergent or convergent light sources can be used with proper adjustment of the optical system to properly align the Fourier transform plane of the image. . Further, the single light source 11
Multiple monochrome light sources can be used instead of zero. These multiple light sources will create more opaque areas on the Fourier filter 120. In other embodiments, illumination can be applied to the wafer in four diagonal directions, in the plane of the wafer surface. These directions are 45 degrees with respect to the pattern line.

In any of the above embodiments, the illumination mask may be used to hide non-repetitive areas of the wafer, such as cut areas. Furthermore, as long as the illumination source 110 and the Fourier filter 120 are held in fixed positions,
The wafer can be scanned (ie, moved relative to other parts of the system) without changing the Fourier pattern. Using this scanning technique, the system can inspect the entire wafer. Although the invention has been described in the context of three embodiments, it can be practiced as outlined above without departing from the scope of the invention.

[0029]

According to the present invention, a defect on a surface having a repetitive pattern can be inspected with high accuracy.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a wafer inspection system according to an embodiment of the present invention.

FIG. 2 is a perspective view that is useful for describing how the inspection system shown in FIG. 1 illuminates a semiconductor wafer.

FIG. 3 is a configuration diagram showing a wafer inspection system according to another embodiment of the present invention.

FIG. 4 is a configuration diagram showing a wafer inspection system according to another embodiment of the present invention.

FIG. 5 is a front view of a Fourier filter used in the wafer inspection system shown in FIGS.

FIG. 6 is a graphical illustration of a spatial spectrum of a semiconductor wafer that is useful in describing the operation of various embodiments of the present invention.

[Explanation of symbols]

 110 helium-neon laser 112 surface mirror 116 wafer 118 Fourier transform lens 120 spatial filter

 ──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-3-102248 (JP, A) JP-A-1-117024 (JP, A) JP-A-1-153943 (JP, A) JP-A-1- 21724 (JP, A) U.S. Pat. No. 4,772,126 (US, A) U.S. Pat.

Claims (4)

(57) [Claims] 1. A method for inspecting defects on a surface having a repeating pattern, comprising: a) illuminating said surface with a monochrome light beam at an angle between 8 degrees and a predetermined maximum angle with respect to the surface of the wafer; b. C) detecting light scattered from the surface at an angle greater than the predetermined maximum angle ; and c) detecting the spatial frequency corresponding to the repetitive pattern as attenuating the spatial frequency corresponding to the defect. Spatially filtering the filtered light; and d) imaging the spatially filtered light such that the image of the defect is enhanced relative to the image of the repeating pattern. 2. The method of claim 1 wherein step a) comprises polarizing monochromatic light. 3. The step (c) includes: performing a Fourier transform of the detected light beam; and filtering the Fourier transformed light beam to attenuate a spatial frequency component corresponding to at least one repetitive pattern. wherein the step d) the method of claim 1, wherein comprises the step of performing an inverse Fourier transform of the filtered light. 4. An apparatus for inspecting defects on a surface having a repetitive pattern, comprising: illuminating means for illuminating said surface with monochrome light at an angle between 8 degrees and a predetermined maximum angle with respect to the surface of the wafer; Lens means positioned with an optical axis perpendicular to the surface for detecting light scattered from the surface at an angle greater than the highest angle of the spatial frequency corresponding to the repetitive pattern as compared to the spatial frequency corresponding to the defect. Filter means for spatially filtering the detected light so as to attenuate the image, and an image for imaging the spatially filtered light so that the image of the defect is emphasized compared to the image of the repeating pattern. Means.